High capacity data networks rely on signal repeaters and sensitive receivers for low-error data transmission. To decode and/or cleanly retransmit a serial data signal, such network components include components for creating a data timing signal having the same phase and frequency as the data signal. This step of creating a timing signal has been labeled “clock recovery.”
Data clock recovery requires a relatively high purity reference signal to serve as a starting point for matching the serial data signal clock rate and also circuitry for frequency adjustment. The type, cost and quality of the technology employed to generate the high purity reference signal varies according to the class of data network applications. For fixed large-scale installations, an “atomic” clock may serve as the ultimate source of the reference signal. For remote or movable systems, components including specially configured quartz resonators have been used. As communication network technology progresses towards providing higher bandwidth interconnections to local area networks and computer workstations, the need has grown for smaller and cheaper clock recovery technology solutions.
For many clock recovery applications, the reference signal generator must be adjustable, i.e. controllable, and then operate on a precisely defined operating curve. This adjustability requirement is conveniently defined as an Absolute Pull Range (APR). APR is defined as the controllable frequency deviation (specified in ±ppm) from the nominal frequency (F0) over a wide range of operating parameters, including frequency tolerance, frequency stability, supply voltage, output load, and time (i.e. aging). Clock recovery may require controllably oscillators having both a minimum and a maximum APR.
For higher frequency applications now in demand, e.g., above 500 MHz, more conventional resonator technologies such as standard AT-cut crystals have not been fully successful. The recognized upper limit for fundamental-mode, straight blank AT-cut crystals is about 70 MHz. Hence, some type of frequency multiplication must be employed to generate the required higher frequency reference signal. With frequency multiplication comes increased circuit sensitivities for phase noise, jitter, non-linearities and long-term stability.
Available alternatives to standard quartz/crystal resonators include the use of surface acoustic wave (SAW) resonators and special crystal blank configurations such as inverted mesa. These alternatives involve more complex manufacturing steps and therefore higher cost.
The focus on cost cutting for data signal clock recovery components is reflected in U.S. Pat. No. 5,987,085 to Anderson. The Anderson patent illustrates a clock recovery circuit developed in an effort to eliminate the crystal-based reference clock requirement. Anderson failed to identify the target frequencies or present operating data, however.
There continues to be a need for a cost-effective voltage controlled crystal oscillator suitable for data signal clock recovery applications. Specifically, it would be desirable to provide a high frequency voltage controlled oscillator utilizing conventional crystal resonators.